The present intention relates to a nonreturn flap valve arrangement, and particularly to a flap valve arrangement which can be used as an inlet valve in a piston-type positive-displacement pump.
European Patent Application No. 89850427.9 discloses piston-type positive-displacement pumps some embodiments of which include a flap valve which functions as a one-way or nonreturn valve between an inlet and a pump chamber which is alternately contracted and pressurised, by means of the displacement element of the pump (the pump piston), for discharging the fluid in the pump chamber through the pump outlet, and relieved from pressure for the purpose of refilling of the pump chamber with fluid through the inlet.
One edge or one end of the valve flap is mounted on one side of a flow passage which opens into the pump chamber, and the other end or opposite edge which is directed towards the pump chamber is movable across the passage into sealing engagement with the other side of the passage under the influence of the fluid forces acting on the flap.
The flap may be devoid of any intrinsic bias, i.e. may be constructed and arranged so as to have no pronounced tendency to adopt any specific position when no fluid-generated forces act thereon, although the flap may, alternatively, be prestressed to a given, relatively slight degree, such that it will tend to take a given, distinct position in the absence of fluid forces thereon and such that the flap will only move from this given position when acted upon by a force that overcomes the bias inherent in the flap.
During the pump-chamber filling phase, the flap is held in a valve-open position by the fluid flowing into the chamber. When the pressure on the outlet side of the valve, or the pump-chamber side, has essentially equalized in relation to the pressure on the inlet side, the flow of fluid into the chamber will cease, and upon commencement of the discharge phase, the pressure on the pump-chamber side will exceed the pressure on the inlet side. The fluid in the pump chamber will then tend to flow back into the flow passage and cause the flap to take a valve-closing position.
Depending on how the flap is constructed and arranged, a larger or a smaller volume of fluid is able to flow back into the flow passage before the flap has closed the passage completely. This backward flow of fluid is disadvantageous and should be limited to the greatest possible extent. At the same time, the flap should be able to open the flow passage quickly at the commencement of the filling phase, so as to expose a large cross-section area for the flow of the fluid into the pump chamber and so that the pump chamber can be filled rapidly with the minimum drop in pressure across the flow passage. The passage should also be as short as possible, for similar reasons, i.e. should have the smallest possible extent in the flow direction. Furthermore, the mass that needs to be accelerated during the filling phase should be as small as possible. This also applies to the mass which needs to be retarded when the inlet valve is closed.
The desirable large cross-section area of the flow passage can be provided by placing the walls defining the flow passage widely apart so that the passage will have the form of a wide gap. A wide gap requires a long flap, however, and also has to be rather long itself, because the flap has to be long enough to bridge the entire gap without having to move through a wide angle to open and close the passage.
A long flap is disadvantageous because in the closed or bridging position thereof a substantial portion of the flap is unsupported by the walls of the passage. During the phase of the pump operating cycle in which the fluid trapped in the pump chamber is discharged, the flap is subjected to high pressure from the fluid in the pump chamber, and when the flap is made of plastic film or plastic foil as is desirable in many cases, the flap will tend to bulge outwardly towards the inlet.
This outward bulging of the flap can be counteracted by reinforcing the flap in some appropriate fashion, although such reinforcement will complicate manufacture and may, to some extent, create undesirable resistance to the opening movement of the flap.